Fix the origin-changes review findings

Hold the 100,000-row cap after a failed write's rows are put back, dropping and counting the oldest rows past it. Stop counts the unwritten origin changes. The prefix handler's per-write origin change limit is a field, so a test checks that one write leaves the rest. The server and README now say where origin changes come from. README, TODO and schema say 72 hours or 500 rows, whichever keeps more. The prefix page and TODO list the origin change columns and say what the peer count counts. The page takes its 5,000-row note from the constant.

Model: deepseek-v4-pro

internal/templates/prefix_detail.html

@@ -297,13 +297,13 @@
                 {{end}}
             </div>
         </div>
-{{end}}
+        {{end}}
 
         {{if .OriginChanges}}
         <div class="origins-section">
             <h3>Origin Changes</h3>
             {{if .MoreOriginChanges}}
-            <p>There are more than 5,000 changes. Showing the newest 5,000.</p>
+            <p>There are more than {{.MaxOriginChanges}} changes. Showing the newest {{.MaxOriginChanges}}.</p>
             {{end}}
             <table class="route-table">
                 <thead>